simple question of current and LED specs that I should probably know...

I suspect this answer should be obvious, but I'm still fairly new at this...

My project requires me to, among other things, light up some LED illuminated buttons from an arduino duemilanove, and I'm concerned about the current they might draw from the board.

The LED lamps I'm using in each button are triple led clusters, seen here: (#91-10WB-53W; 5-6v; white; triple led cluster; T 3-1/4)

I emailed the LED supplier for the specs, and they returned with this:


I know each arduino pin can output a max of 40mA, but I'm not quite sure what to make of some of these LED specs. I'm inclined to read those specs to mean that it operates normally at 20mA, which is good for me. The 60mA is the maximum the bulb withstand continuously, and 180mA is the maximum it can take when pulsed? But the bulb won't draw that much current unless it's supplied, correct? So... a single one of these should operate fine powered off a single pin? I just don't want to burn out my arduino.

The safe answer, I suppose, would be to use a transistor to switch each bulb indirectly, powering them all from the 5v pin. But it'd be nice to control things directly if safe.

Thanks in advance for any help!

I'm surprised the supplier didn't say anything about forward voltage. I think it's safe to source 20mA continuously from Arduino pins (but not all at once!) and LED's are commonly specified at this current, but you can definitely operate them at higher or lower currents.

I think it's OK to power 3 LED's (i.e., 3 I/O pins) at 20mA but you should use a series resistor to limit the current and drop the voltage to the typical forward voltage of the LED's, which the manufacturer didn't specify. Yeah, the part says "6V" but who knows at what current.

-- The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

Thanks for that, and so quickly too!

The product page does indicate the LEDs are equipped with a resistor, but they kindly don't say anything more about that.

hmmm actually, reading that comment again, I’m not sure it does answer my question…

think it’s OK to power 3 LED’s (i.e., 3 I/O pins) at 20mA

except, it’s not three LEDs on 3 I/O pins… it’s a single lamp that contains a 3 LED cluster, and that single lamp would be running off a single I/O pin.

additionally, the spec sheet does say the lamp includes a resistor (though sadly it doesn’t say what value it is). I was hoping there was something in the spec sheet I was missing that would indicate whether the lamp ever pulls more than 20mA, but it might just be that it doesn’t give enough info to answer my question.

I did (perhaps foolishly) hook the LED up to the board, and it works fine using the blink test. I tried getting an amp read using my multi-meter, but it doesn’t seem to work… The meter reads 0 and the LED won’t light up when I attach the meter in series. The LED does work if I just hook it up by itself.

The meter reads 0 and the LED won't light up when I attach the meter in series.

Did you remember to switch the probe lead to the current measuring position? :) I do that all the time.

OK: a single I/O pin sourcing 20mA is OK. A single I/O pin sourcing 20mA each into three separate LED's, or 60mA, is NOT OK.

Try to get that DMM working and that will solve everything! You can also look at the pin voltage. If it's drastically far away from 5V then it's working too hard and putting out too much current.

-- The Flexible MIDI Shield: MIDI IN/OUT, stacking headers, your choice of I/O pins


Are you sure, power is measured in watts so 300mW is more like. You can't drive this off a single arduino pin you need a transistor.

RuggedCircuits~ Yup - the dmm is set to measure dc current. I even tried another meter. I must be doing something wrong, but god knows what. You measure current in series, right? (PIN---Red Prong---Meter---Black Prong---LED---Ground) And measuring the voltage in parallel, btw, the pin drops about 1v when the LED is lit.

But all this aside, I'm thinking the best thing to do it just power the LEDs from the 5v or VIN pin and not worry about all this stuff. Thank you for your valiant attempt in the face of missing info and a beginner's mind.

This is a basic simon-like game using 5 buttons and 5 of these LEDs. I doubt they'd be turned on all at the same time, but still, I'm thinking I'll power them from the VIN rather than the 5v, since the current rating is higher and it'll avoid the question. I'm using one of these 5v regulated wall warts (from sparkfun - - max current of 1amp. Now, since these LEDs are spec'd as 5-6v and include the necessary resistor, I should just be able to hook them up directly to the VIN right? I might not know the forward voltage, but they did when they built in the resistor...

Grumpy_Mike~ I just copied/pasted what they sent. I would assume you're right and they meant mW not MV. Thanks - That was plan B and I think I'm just going to do that to play it safe, and easy.

I must be doing something wrong, but god knows what. You measure current in series, right? (PIN---Red Prong---Meter---Black Prong---LED---Ground)

It's likely at some former time you blew the fuse that protects the current function for the meter. It's easy to do (ask me how many times I've done it over the decades) if you have your meter leads in the current position but then measure a voltages = short circuit through the meters current shunt and protection fuse. Figure out where the fuse is accessed from in your meter, remove it and test it with a ohm reading.


It's likely at some former time you blew the fuse

Lefty- that was it! thanks!

(PIN---Red Prong---Meter---Black Prong---LED---Ground)

You need a resistor in there as well.